Organ-at-risk sparing with dynamic trajectory radiotherapy for head and neck cancer: comparison with volumetric arc therapy on a publicly available library of cases

Background Dynamic trajectory radiotherapy (DTRT) extends volumetric modulated arc therapy (VMAT) with dynamic table and collimator rotation during beam-on. The aim of the study is to establish DTRT path-finding strategies, demonstrate deliverability and dosimetric accuracy and compare DTRT to state-of-the-art VMAT for common head and neck (HN) cancer cases. Methods A publicly available library of seven HN cases was created on an anthropomorphic phantom with all relevant organs-at-risk (OARs) delineated. DTRT plans were generated with beam incidences minimizing fractional target/OAR volume overlap and compared to VMAT. Deliverability and dosimetric validation was carried out on the phantom. Results DTRT and VMAT had similar target coverage. For three locoregionally advanced oropharyngeal carcinomas and one adenoid cystic carcinoma, mean dose to the contralateral salivary glands, pharynx and oral cavity was reduced by 2.5, 1.7 and 3.1 Gy respectively on average with DTRT compared to VMAT. For a locally recurrent nasopharyngeal carcinoma, D0.03 cc to the ipsilateral optic nerve was above tolerance (54.0 Gy) for VMAT (54.8 Gy) but within tolerance for DTRT (53.3 Gy). For a laryngeal carcinoma, DTRT resulted in higher dose than VMAT to the pharynx and brachial plexus but lower dose to the upper oesophagus, thyroid gland and contralateral carotid artery. For a single vocal cord irradiation case, DTRT spared most OARs better than VMAT. All plans were delivered successfully on the phantom and dosimetric validation resulted in gamma passing rates of 93.9% and 95.8% (2%/2 mm criteria, 10% dose threshold). Conclusions This study provides a proof of principle of DTRT for common HN cases with plans that were deliverable on a C-arm linac with high accuracy. The comparison with VMAT indicates substantial OAR sparing could be achieved. Supplementary Information The online version contains supplementary material available at 10.1186/s13014-022-02092-5.

Optimization objectives for the OARs are also set before the optimization based on the clinical goals (Table A.II) but with a priority 0. For the phase 2 and 3 plans, the dose is reduced proportionately to the ratio of prescription dose (e.g. 32% for phase 2 delivering 16 Gy). The objectives in Table A.II are presented in order of priority. Only the spinal cord, brain stem and their respective PRVs have higher priority than target coverage. For all other OARs, if the clinical goals cannot be met, the dose should be reduced as much as possible. The PRV margins were 5 and 3 mm isotropically for spinal cord and brain stem respectively.
Carotid arteries are only considered if outside the target volume with an isotropic PRV margin of 5 mm.  3 : 20 Gy if only one gland can be spared. Keep below 30 Gy for at least one gland. 4 : Optimization is performed on oral cavity but plan evaluation is performed on oral cavity excluding PTV

Interactive optimization
Optimization is started in Eclipse with only the objectives of table A.I with non-zero priority. No interaction is permitted until the PO reaches MR level 1, step 3/5. The user then pauses the optimization and sets a priority of 80 to all OAR objectives. Exceptions are made for OARs that have a minimum dose objective (Upper objective to 0% volume) and overlap with the PTV or for OARs with a mean dose objective having a major overlap with the PTV. In this case, the dose value is set to the prescription dose and the priority to 50.
The user resumes the optimization and is allowed only to change the dose value of the OAR objectives. The goal of these interactions is to fulfil the clinical goals and to reduce the dose to the OARs as much as possible, i.e. the objectives must contribute to the objective function value. After the intermediate dose calculation at MR level 4, the user once again pauses the optimization and can make final adjustments to the dose values of the OAR objectives. The user resumes the optimization again until it completes. Final dose calculation is performed and the user evaluates whether the clinical goals are fulfilled.
Tweaking the objective and re-optimization without manual interaction was allowed if clinical goals were nearly reached. All plans were re-optimized without any manual interaction and the plans were normalized.
A.II. Film measurement protocol Film measurements were conducted in one end-to-end test on the Alderson phantom. Two Gafchromic EBT3 films (Ashland Advanced Materials, Bridgewater, NJ) were laser-cut to fit between the Alderson slabs closest to target isocenter. The films were scanned 21 hours after irradiation on an Epson XL 10000 flatbed scanner. The scanned films were corrected for lateral response artefact using a one-dimensional linear correction function [2]. Triple channel calibration was used to convert colour values to absolute dose [3]. Dose rescaling was applied according to the one-scan protocol using two addition film strips [4]. The resulting dose to the red channel was used for comparison with the corresponding 2D calculated dose plane in Eclipse using gamma evaluation with a 2% (global)/2mm criterion and a 10% dose threshold of the maximum dose [5].
A.III / A.IV DVH endpoints Table A.III reports clinical goals and DVH endpoints for HN1-6 treated with sequential boost techniques. Table A.IV reports clinical goals and DVH endpoints for HN7, single vocal cord irradiation treated according to the VoiceS protocol (NCT04057209, available upon request). Endpoints above tolerance are shown in red. As per institutional guidelines, these clinical goals are not mandatory but the dose should be reduced as much possible and the following variations are acceptable: -Parotid glands: mean dose below 30 Gy for at least one gland, -Pharynx: mean dose below 55 Gy, -Oral cavity (excluding PTV): mean dose below 50 Gy, -Brachial plexus and brain: D0.03cc below 70 Gy -Mandible: D2% below 75 Gy

A.V RATiNG
The RATiNG guidelines for treatment planning studies [6] were followed and the final score was 96%. The score sheet is attached as supplementary material.
Supplementary Figure 1: Individual OAR GT-maps and map sum for each path for HN2. The hippocampus R+L was included in both maps.